The technique of water purification supplied by membranes has an increasing number of industrial implementations of all capacities. The fields of membrane filtration techniques overlap but are generally divided into:                microfiltration, in the region of pore diameters of approximately 0.5 μm,        ultrafiltration, in the region of pore diameters of approximately 10 nm,        nanofiltration, having a cut-off capability of around 1 nanometer or 250 daltons (in practice from 200 to 300 daltons for nanofiltration membranes currently on the market),        reverse osmosis below 1 nanometer.        
Nanofiltration makes it possible to eliminate a substantial proportion of the dissolved organic pollution and certain mineral ions. More particularly, it enables soluble organic minerals and certain mineral ions of quite large “apparent” size. On account of this, nanofiltration overlaps the field of reverse osmosis. Nanofiltration is also referred to as “low pressure reverse osmosis” or “hyperfiltration”.
Due to its very low cut-off thresholds, nanofiltration ensures total microbial sterility.
Nevertheless, in industrial installations, nanofiltration membranes are mounted within nanofiltration modules, and these nanofiltration modules are themselves implemented by virtue of arrangements of a high number of modules capable of ensuring the required throughput. Thus, even though nanofiltration is inherently impermeable to bacteria and viruses at membrane level, the arrangement of a high number of modules may include connection defects which put in doubt the impermeability of the modules.
Furthermore, the possibility always exists of modules that are defective or badly constructed (poor glued joints of the membranes, holes not detected in the membrane, etc.).
There are two general configurations commonly used to mount nanofiltration membranes within a module:                a so-called “spiral” configuration implementing flat membrane sheets, separated by spacers, rolled around a tube collecting the filtrate (that is to say the part of the fluid to be treated which has passed through the membranes, in contrast to the concentrate which is the part of the fluid which has not passed through these membranes and in which are concentrated the impurities held back by the membranes.        a so-called “hollow fiber” configuration, in which the membranes appear in the form of bundles of capillary fibers.        
Whatever the type of configuration, the modules (in general of a diameter of 8 inches and a length of 40 inches for industrial water treatment installations) are connected to each other, in principle in series, within so-called “pressure tubes”. A complete nanofiltration system may comprise several pressure tubes mounted in parallel, conventionally fixed onto units (or skids), or even most often several units (or stages) each formed by a plurality of tubes.
The problem which arises in practice is that of detecting a defect in the integrity of a module, or of a system constituted by a plurality of modules, of which the origin lies either with the membrane themselves, or with the joints, and which results in a passage of the fluid directly from the dirty side of the membrane (concentrate) to the clean side (filtrate) through the pierced membrane or deficient joint.
Thus there are two main categories of checking methods applied to models taken individually, that is to say methods of detecting the modification of the integrity of modules for nanofiltration or reverse osmosis.                methods implementing quality control of the water produced, i.e. of the filtrate (in particular by measurement of conductivity, or by bacteriological analyses),        methods implementing leak detection using a physical process (for example measurement of throughput, or measurement of low pressure).        
These methods are implemented on the filtration modules taken individually and generally placed on test-beds; they cannot be used to locate defects in connections within a system in operation.
The method may also be cited of leak detection by noise measurement proposed in document WO99/44728, but this method is not applicable to reverse osmosis nor to nanofiltration, the latter two methods requiring operation in tangential flow mode of the membranes. This is because this mode uses higher pressures and speeds, and consequently the noises of leaks cannot be distinguished from ambient noise.
Methods also exist which apply to overall systems of modules; these methods in principle use measurements of conductivity of the water produced by the systems. Thus, for the desalination of sea water by reverse osmosis, a measurement of conductivity is generally sufficient. However, for nanofiltration membrane systems, the measurement of conductivity of the water produced by the systems does not enable defects to be detected with the necessary precision. This is because, by nature, nanofiltration lets through a high number of salts such as calcium, chlorides, nitrates, etc. without this being attributable to a defect in the integrity of the system.